Contract account locking issue during FPB7 run

Scenario # I have incoming payment and refund in the lock box file for the same Contract account with buyer invoice and buyer refund FICA open documents. When i have executed the lock box program i have generated two different sort keys which are processed in parallel through FPB7. When FPB7 got executed successfully one lot with invoice payment posted successfully but lot with refund wen in to clarification because of Contract account locking issue.

Is there any way i can avoid this locking issue while running FPB7 in parallel. My clearing is happening with document number as selection in payment lot not through contract account.